Field of the Invention
The present invention relates to a wavelength selective switch.
Related Background Art
A wavelength selective switch is described in Japanese Patent No. 4445373. The wavelength selective switch includes a collimator array to be an input/output optical system, a spectroscope to disperse input wavelength multiplexing light to a wavelength component, a condensing lens to be a condensing optical system, and a micro mirror array unit to be a switching element. As the spectroscope, a diffraction grating is used. The micro mirror array unit is a micro mirror that is arranged in an array.
A wavelength selective switch is described in Japanese Patent No. 4500720. The wavelength selective switch includes a collimator array to be an input/output optical system, a beam expander, two diffraction gratings, a condensing lens, and a micro mirror array unit.
A spatial position of a wavelength component condensed by the condensing lens depends on a wavelength non-linearly. This is because a dispersion angle of each wavelength component given by the diffraction grating changes non-linearly for a frequency (wavelength). Meanwhile, in normal wavelength multiplexing optical communication, a frequency of each wavelength component is defined at an equal interval. For this reason, in the case in which micro mirrors are arranged at an equal interval in a dispersion direction of the wavelength component, if a specific wavelength component is adjusted to be incident on the center of the micro mirror, a different wavelength component is shifted from the center of the micro mirror and is incident on the micro mirror and thus, a transmission band is deteriorated.
In order to resolve such a problem, it is considered that micro mirror array units arranged at an unequal interval are used such that an interval of the micro mirrors is matched with the spatial position of the wavelength component. Thereby, each wavelength component is incident on the center of each micro mirror.
However, it is difficult to manufacture the micro mirror array unit and a manufacturing cost is high. In addition, when a spatial light modulator such as LCOS is used as a switching element, instead of the micro mirror array unit, complex control of a phase modulation pattern is necessary.
In addition, a wavelength component dispersed by a first diffraction grating transmits a second diffraction grating, so that a dispersion angle can be doubled. At this time, a length of an optical path of each wavelength component until each wavelength component is emitted from the first diffraction grating and is incident on the second diffraction grating is different and an incidence angle of each wavelength component on the second diffraction grating is also different.
For this reason, the reverse trace of an optical path of each wavelength component emitted from the second diffraction grating is not matched at a focal point of the condensing lens. Thereby, each wavelength component emitted from the condensing lens is incident on the micro mirror array at a different angle. As a result, coupling efficiency of each wavelength component in the input/output optical system is deteriorated.
In order to resolve such a problem, it is considered that each micro mirror is tilted according to a tilt of an incidence angle of each wavelength component on the micro mirror array. However, control of the micro mirror array becomes complicated. Even when the spatial light modulator is used as the switching element, the complex control of the phase modulation pattern is necessary.